z80_unpacker: optimisation: -2B in backward unpack (fwd 172B / rev 169B)

backward was already -1B, so now the total difference is -3B.

z80_unpacker/unpack.asm

@@ -139,16 +139,19 @@ unpack:
     call    decode_number       ; length = upkr_decode_length(257 + 64);
     push    de
     exx
+    IFNDEF BACKWARDS_UNPACK
         ld      h,d             ; DE = write_ptr
         ld      l,e
 .offset+*:  ld  bc,0
-    IFNDEF BACKWARDS_UNPACK
         sbc     hl,bc           ; CF=0 from decode_number ; HL = write_ptr - offset
-    ELSE
-    add     hl,bc               ; HL = write_ptr + offset
-    ENDIF
         pop     bc              ; BC = length
-    IFNDEF BACKWARDS_UNPACK : ldir : ELSE : lddr : ENDIF
+        ldir
+    ELSE
+.offset+*:  ld  hl,0
+        add     hl,de           ; HL = write_ptr + offset
+        pop     bc              ; BC = length
+        lddr
+    ENDIF
     exx
     ld      d,b                 ; prev_was_match = true
     djnz    .decompress_data    ; adjust context_index back to 0..255 range, go to main loop